1. Field of the Invention
The present invention generally relates to a method and apparatus for acoustic leak detection, and more particularly to an improved method and apparatus for quickly and accurately locating the source of a leak in a pipeline utilizing pattern match filtering techniques.
2. Related Art
In pressurized systems, such as pipelines, the pressure boundary is maintained by the pipe wall. The pipe wall has a yield stress greater than the stress exerted from the system pressure. At the instant of a breakdown of the pipe wall, the release of the elastic force couples with the system fluid to create a transient pressure wave. Since pressure is relieved (due to the break in the pipe wall) from the containment system, the transient pressure wave takes the form of an expansion wave. This expansion waves travel outward in all directions from the source at the speed of sound for that fluid. When the fluid is contained in a pipeline, the expansion waves are guided through the fluid by the walls of the pipe in either direction from the source of the break.
Real time acoustic pipeline leak detection requires placing permanent monitors on a pipeline for detecting expansion pressure waves associated with a sudden break down of the pressure boundary due to a rupture in the pipe wall. In the past, it was the amplitude of the signal that was of concern. The source of the pressure waves was located between monitors by recording the time when the expansion pressure wave arrived at the monitors. Using these times (t2 and t1), knowing the fluid sound velocity (V) and the length of pipe between monitors (D) the leak event could be located. As shown in the following equation where X is the leak event location (distance from sensor number 1):
X=D/2+V(t2xe2x88x92t1)/2 
If this event was not located between the monitors, it was ignored as a false event. The problem encountered with this method is that multiple transients on the pipeline from normal operating procedures can produce pressure disturbances (similar to an expansion wave generated by leak event) which are located on the monitored segment of pipeline and thus cause false leak alarms. Previously, background noises were filtered out by various techniques such as moving average, repetitive filter, dynamic threshold and band pass filters. Although, these have successfully filtered out certain types of background noises, these techniques have little affect on other types of transient noises, such as noises from pumps, compressors, and valve operations. These transient noises oftentimes produced signals with amplitudes similar to the amplitudes of such signals produced by leaks. This has led to a high false alarm rate and reduced sensitivity.
Additionally, knowing the exact time of arrival of the expansion pressure wave at each monitor is critical for precisely locating the leak on the pipeline. Since each monitor must record the moment the expansion wave arrives at its location, it is critical that each monitor records time on the same part of the expansion pressure wavefront. The low frequency content of the expansion pressure wave produces a wavelength of from a few hundred meters to 100,000 meters or more. Because of the length of the wavefront and other factors, an uncertainty in the time tag between monitors occurs, which in the past has limited leak location accuracy to typically +/xe2x88x92500 or more meters out of 10,000 meters. This has led to the inability to accurately locate leaks.
Accordingly, what is needed, and has heretofore not been provided, is a method and apparatus for pipeline leak detection which can filter out transient noise and thereby decrease false alarms while providing increasing sensitivity and accuracy. Additionally, what is needed, as not heretofore been provided is a method and apparatus for accurately determining the time of occurance of a leak to aid in accurately locating the leak. Previous efforts in detecting and locating pipeline leaks include:
Previous efforts in this area include:
U.S. Pat. No. 3,760,280 to Covington, which discloses a method and apparatus for delaying an electrical signal. The electrical signal to be delayed is converted into a frequency modulated signal which is coupled to a digital memory device that operates in response to a control signal. The rate of the control signal and the capacity of the memory device determine the delay of the FM signal. The delayed FM signal is then demodulated back to its original format.
U.S. Pat. No. 3,903,729 to Covington, discloses a method and apparatus for detecting a break or other occurrence in a pipeline containing gas under pressure. This patent discloses detecting the adiabatic pressure wave generated in the gas by the break and propagated through the gas at the speed of sound. The location of the break is determined by the change of pressure with respect to time of the leading wedge of adiabatic pressure wave. Spaced pressure-electrical transducers are utilized to detect the compressional waves. Electronic circuitry is utilized to delay a selected one of the transduced electrical signals for a selective time interval to substantially eliminate the portion of the signal responsive to compressional wave traveling in the direction opposite the preselected direction.
U.S. Pat. No. 4,455,863 to Huebler, et al., discloses the sonic detection of gas leaks in underground pipes. The patent detects sound waves created by leaking gas using a sound transducer attached to an elongated probe inserted into the ground for a substantial portion of its length. The elongated probe and transducer combination has an effective mechanical resonant frequency equal to or below the electrical resonant frequency of the sound transducer. The passive sonic detection apparatus and process of this invention provides improved sensitivity for detection of sounds created by leaking gas and thereby more accurate pinpointing of the gas leak in an underground pipeline.
U.S. Pat. No. 5,101,774 to Marziale, et al., discloses an acoustic leak detection system. The system is monitored for leaks by an acoustic leak detection system responsive to atmospherically carried sound transmissions. Energy level amplitudes of respective analog electrical signals generated sequence multiplicity of microphones are converted in a rapid time sequence to a first electric pulse signal sequence represented of a corresponding digital values.
U.S. Pat. No. 5,201,212 to Williams, discloses a method and apparatus for testing underground fluid containing lines for leaks. The apparatus includes a differential pressure transducer mounted to a reservoir for indicating volumetric change in the reservoir, a temperature transducer mounted in the reservoir for monitoring temperature fluctuation in the reservoir, a gauge pressure transducer mounted in the reservoir, and a remote temperature sensor and data acquisition and processing system. Readings are taken and pressure and temperature fluctuations in the line are tested at 30 second intervals. Thereafter, the system calculates the leak rate during each 5 minute interval of the test, as well as a cumulative leak rate.
The present invention addresses the problems and limitations of the previous efforts of others by comparing not only the amplitude of signals, but also the wave shape, against previously recorded pipeline leak expansion waves generated by real leak events in pipelines both in the field and by experimental testing. The present invention provides a method and apparatus for filtering the undesirable transient wave signals, while detecting the expansion waves signals due to a break in the wall of the pipe. Additionally, the present invention provides a method and apparatus for specifically determining the time when a leak occurs so that it can be accurately located.
None of the previous efforts of others, taken either alone or in combination, teach or suggest all of the elements of the present invention, nor the advantages and benefits of the present invention.
It is a primary object of the present invention to provide a method and apparatus for detecting and locating leaks in pipelines which has increased sensitivity and increased accuracy.
It is an additional object of the present invention to provide a method and apparatus for detecting and locating leaks in pipelines which reduces false alarms.
It is even an additional object of the present invention to provide a method and apparatus for detecting and locating leaks in pipelines which uses pattern match filtering.
It is still even an additional object of the present invention to provide a method and apparatus for detecting and locating leaks in pipelines which uses pattern match filtering to discriminate against background noise and pressure disturbances generated by other non-leak sources.
It is yet an additional object of the present invention to provide a method and apparatus for detecting and locating leaks in pipelines which continuously compares previously recorded leak profile data with current pipeline data collected in real time to detect leaks in the pipeline.
It is even yet an additional object of the present invention to provide a method and apparatus for detecting and locating leaks in pipelines by deriving the sharp peaked output of pattern match filter associated with a leak signal to provide a distinctive reference point for use as a time stamp for improving accuracy in leak location calculations.
It is still a further object of the present invention to provide a method and apparatus for detecting and locating leaks in pipelines which utilizes site processors located along a pipeline for making comparisons, and which also utilizes node processors to make further comparisons to thereby further reduce false alarms.
A method and apparatus is provided for detecting and locating leaks in a pipeline. Pattern match filtering is used to reduce false alarm rate, increase sensitivity and improve leak location accuracy, while quickly detecting leaks by the acoustic signal generated from a leak event in pipelines containing either gas or liquid or both (two phase flow) under pressure. The pattern match filter technique detects a pressure wave generated by a leak, but discriminates against background low noise and pressure disturbance generated by other non-leak sources that might otherwise be detected as a leak. The pattern match filter derives a distinctive point of reference from the signal of the expansion wave which allows for an accurate time stamp. This provides for improved accuracy in leak location calculations. The pattern match filter is incorporated into site processors located at multiple points along a pipeline, and at a central node processor which receives data from all site processors. The pattern match filter includes using previously recorded leak profiles. At site processes located at multiple points along a pipeline, a series of previously recorded signature leak profiles are continuously compared in real time against pipeline pressure signals. Data from each site processor are used collectively at a node processor and compared against multiple leak profiles to provide further false alarm rejection. The leak event data generated at each site processor is used by the node processor to declare a leak. By the application of this pattern match filter technique, the signal to noise ratio (S/N ratio) required to identify a leak event is reduced and the sensitivity of leak detection is increased.